Reciprocatory motors

ABSTRACT

A reciprocatory fluid pressure operated motor in which a piston having a cam profile on its peripheral surface is axially reciprocated in a piston cylinder and a spool with its axis disposed radially with respect to the piston axis is biased through its spool cylinder to abut the piston and ride over the cam profile during axial reciprocation of the piston by which movement is imparted to the spool along its axis, and in which thrust means, preferably in the form of hydrostatic pads, is provided between the piston and its cylinder to oppose the radially directed force applied on the piston through the spool from the biasing means associated with the spool and maintain the piston in balance.

United States Patent [72] Inventor Philip Butterworth Bramhall, England 2| Appl. No. 795,535 [22] Filed Jan.3l,l969 [45] Patented July 27, 1971 [73] Assignee Butterworth Hydraulic Developments Limited [32] Priority Feb. 12, 1968 [33] 7 Great Britain [31 1 6795/68 [54] RECIPROCATORY MOTORS 3,338,140 8/1967 Sheesley 92/138 614,073 11/1898 Ball 91/348 1,020,003 3/1912 Williams 91/ 348 FOREIGN PATENTS 1,200,733 7/1959 France 92/138 Primary Examiner-Paul E. Maslousky Attorney-Stevens, Davis, Miller & Mosher ABSTRACT: A reciprocatory fluid pressure operated motor in which a piston having a cam profile on its peripheral surface is axially reciprocated in a piston cylinder and a spool with its axis disposed radially with respect to the piston axis is biased through its spool cylinder to abut the piston and ride over the cam profile during axial reciprocation of the piston by which movement is imparted to the spool along its axis, and in which thrust means, preferably in the form of hydrostatic pads, is provided between the piston and its cylinder to oppose the radially directed force applied on the piston through the spool from the biasing means associated with the spool and maintain the piston in balance.

PATENTEU m2? I971 SHEET 1 BF 2 :VVV. m

PATENTED JUL27 usn 595; 134

sum 2 OF 2 RECEPROCA'EORY MOTORS This invention relates to reciprocatory motors and in particular to such motors of the type in which a piston is adapted to reciprocate intermittently or continuously in a piston cylinder by fluid pressure operation.

It is an object of the present invention to provide a reciprocatory motor of the type described in which reciprocation of the piston is adapted to impart axial movement to a spool mounted in a spool cylinder, said axial movement being in a direction which is substantially radially disposed relative to the piston.

According to the present invention there is provided a fluid pressure operated reciprocatory motor which includes a piston adapted to reciprocate intermittently or continuously in a piston cylinder; a spool capable of reciprocatory movement in a spool cylinder; the spool cylinder being fixedly located with respect to the piston cylinder with its longitudinal axis disposed substantially radially relative to the piston cylinder and the spool cylinder communicating with the piston cylinder, wherein the piston is provided with a cam surface and the spool is provided both with an abutment surface which is adapted to abut the cam surface and with biasing means which is adapted to urge the spool axially and the abutment surface into engagement with the cam surface, whereby during reciprocation of the piston the abutment surface rides over the cam surface to impart axial movement to the spool.

It will be apparent that the direction of axial movement of the spool is dependent on the profile of the cam surface and the direction of axial movement of the piston and that the speed of axial movement of the spool will relate to the speed of axial movement of the piston and the profile of the cam surface. By providing a suitable profile to the cam surface a required locus can be obtained for axial movement of the spool relative to the axial movement of the piston.

Conveniently the spool cylinder and piston cylinder are defined by a common housing and at least the spool is provided with a rod which extends from the housing. Particularly for machine tool applications, the spool rod (or the piston rod as convenient) can be fixedly secured to a frame or head of the machine and a tool (for erample a cutter) attached to the piston rod (or to the spool rod in the case where the piston rod is secured to the frame) so that on reciprocation of the piston the tool exhibits a desired compound motion which results from reciprocation of the piston rod in the housing and axial movement of the housing on the spool.

Fluid pressure operated reciprocatory motors of the piston and cylinder type are frequently used to obtain controlled axial movement of a piston rod (and apparatus coupled thereto). Such motors have generally proved satisfactory in providing controlled movement of a piston over long distances but it has been found that controlled movement of a piston over relatively short distances, say in the order of thousandths of an inch, is very difficult to achieve with efficiency. This is primarily due to, what is known in the art as, "stiction which is basically frictional forces developed between the piston and its cylinder during initial movement of the piston in its cylinder and the adhesive effect of a film of fluid which is usually present between the adjacent sliding walls of the piston cylinder. As a result of stiction, initial movement of the piston under fluid pressure is usually erratic and as the length of controlled movement of the piston is decreased to the order of fractions of an inch the effect of stiction becomes an important factor whereas for longer controlled movements of the piston the efl'ect of stiction can usually be neglected. By the motor of the present invention, controlled movement can be taken from the spool which movement is dependent on the profile of the cam surface. COnsequently the motor can act as a gear system so that, by providing a linear cam surface which is inclined relative to the axis of the piston, very small axial movement can be transmitted to the spool for very large axial movement of the piston and thereby the etTect of stiction between the piston and its cylinder can be neglected. For example, if the cam surface is linear and is inclined to the axis of the piston in the ratio of l:l00, for 1 inch controlled movement of the piston, the spool exhibits controlled movement through ten-thousandths of an inch. The motor of the present invention therefore affords considerable advantages in its application to the provision of small controlled movements in conditions where the only convenient source of power is fluid under pressure, for example in the adjustment and control of scientific instruments.

As above mentioned, the motor of the present invention can act as a gear system and by providing a predetermined relationship between the cam and abutment surfaces a required mechanical advantage can be obtained between the input to the piston and output of the spool. For example, in the aforementioned case in which the cam surface has an inclination of 1:100 the mechanical advantage which is afforded is considerable and, for example, the common piston and spool housing can be secured to a fixed frame and relatively heavy loads can be moved through predetermined distances relative to the frame by controlled movement of the spool.

Under the action of the biasing means, (and possibly axial loading on the spool), the abutment surface of the spool applies a radially directed force onto the piston which force could, in the absence of the thrust means, cause the piston to be displaced radially in its cylinder and thereby the frictional forces developed between the piston and its cylinder to be in creased on the side of the piston remote from the position of engagement between the abutment surface and the cam surface. ln the absence of the thrust means such radial displacement of the piston would increase the friction characteristics between the piston and its cylinder, possibly to such an extent that the piston is effectively siezed and cannot be reciprocated under normal conditions of use. By the present invention, the thrust means ensures that the piston is free to reciprocate irrespective of the radial force exerted on it by the biasing means through the spool.

in a preferred form of construction the biasing means is provided by the action of fluid under pressure on the spool. The spool is provided with a working face which communicates with a constant pressure spool chamber so that when fluid under pressure is passed into the constant pressure spool chamber the spool is biased to urge the abutment surface into engagement with the cam surface by the effect of fluid pressure on the effective working pressure area of the working face. By the effective working pressure area we mean the ef fective area of the spool on which the fluid pressure acts to bias the spool in one sense of direction, for example, the spool can be stepped down to provide an annular shoulder which communicates with the constant pressure spool chamber and the part of the spool of smaller diameter provides a spool rod which extends through the constant pressure spool chamber and the housing which defines it, in this case the effective working pressure area is the area of the annulus formed between the two diameters of the spool. The thrust means to maintain the piston in balance is preferably provided by pressure fluid pad means which is located in recess means provided between the piston and its cylinder.

ln a preferred arrangement the thrust means comprises one fluid pad which is located on the side of the piston remote from the position of engagement between the cam surface and abutment surface. Tl-le fluid pad can conveniently be formed in a recess in the piston cylinder which is substantially in axial alignment with the axis of the spool cylinder. Alternatively the fluid pad can be formed in a recess in the piston which recess is substantially in diametral alignment with the cam surface of the piston.

Conveniently the recess means in which the fluid pad or pads of the thrust means are formed communicates with the constant pressure spool chamber so the pressure of fluid in the pad or pads is substantially equal to the pressure of fluid in the constant pressure spool chamber and in the preferred arrangement, where one pad is provided which is substantially diametrically opposed to the abutment means, the area of the pad is arranged to be substantially equal to the effective working pressure area of the spool so that the substantially diametrically opposed forces on the piston are equal and the piston is maintained in balance.

The piston and cylinder is conveniently a fluid operated motor of the kind in which the piston has opposed working faces of different effective areas. The smaller effective area communicates with a constant pressure piston chamber which is adapted to be in permanent communication with a source of fluid under constant pressure to bias the piston in one sense of axial direction and the larger effective area communicates with an alternating pressure piston chamber which is adapted connected through control means to alternately communicate with the source of fluid under constant pressure and with exhaust so that when the alternating pressure piston chamber is open to exhaust the piston moves in the direction of its bias and when the alternating pressure piston chamber communicates with fluid under pressure the piston moves in a direction against its bias. A fluid motor of the kind above described is disclosed in each of our copending British Pat. applications Nos. 9,858/65 (1,142,852), 28,148/67, 41081/67, 41082/67 and 55226/66, and the present invention can be incorporated in each of said fluid motors. Recess means for the fluid pad or pads which provide the thrust means and the constant pressure spool chamber can conveniently communicate with the constant pressure piston chamber. Either a passage can be provided in the piston which communicates between the constant pressure piston chamber and the recess means in which the fluid pad or pads are located and formed in the peripheral wall of the piston or a passage can be provided in the housing which defines the piston cylinder which passage communicates between the constant pressure piston cylinder and recess means in the piston cylinder in which the fluid pad or pads are located and formed in the wall of the piston cylinder.

in cases where the fluid pad or pads are formed in recess means in the piston, means is provided to prevent the piston from exhibiting axial rotation in its cylinder. Such means is conveniently achieved by suitably forming the cam surface and abutment surface. The cam surface can be formed having the required profile in the axial sense of the piston but being flat in transverse (diametral) section through the piston so that, in effect, the cam surface is a chord of the circular form of the piston.

The abutment surface can be provided by a roller bearing carried by the spool for example a sphere, cylindrical roller, needle bearing or the like which is adapted to rotate as it moves over the cam surface.

it is to be understood that the term fluid as used throughout this specification includes both liquids and gases, for example the invention may apply equally to pneumatic (gas-pressure-operated) motors and to hydraulic (liquid-pressure-operated) motors, the detailed modifications necessary for the particular type of fluid being obvious to a person skilled in the relevant art.

One embodiment of the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which:

FIG. 1 is an axial section through a reciprocatory motor constructed in accordance with the present invention in which a recess for the location of a fluid pad to provide thrust means is situated in the peripheral surface of the piston;

FIG. 2 is a transverse section of the motor shown in FIG. l and is a section taken on the line 11-11 of F 1G. 1; and

FIG. 3 illustrates a modification of the motor shown in FIG. 1 in which a recess for the location of a fluid pad to provide thrust means is situated in the piston cylinder.

Where possible throughout the following description the same parts or members in each of the Figures have been accorded the same references.

Referring firstly to F268. 2 and 2, the reciprocatory motor illustrated comprises a stepped piston l which is axially slideble within a stepped piston cylinder 2 defined by a housing 3. The smaller diameter of the piston 1 slides within the smaller bore of the piston cylinder 2 and the larger diameter of the piston slides within the larger bore of the piston cylinder. The piston l is provided with a piston rod 4 which extends axially in sealed manner through the housing 3.

The ends of the piston 1 provide opposed working faces having different effective working pressure areas, the smaller effective working pressure area 5 defines with one end of the piston cylinder 2 a constant pressure piston chamber 6 and the larger effective working pressure area 7 defines with the other end of the piston cylinder 2 an alternating pressure piston chamber 8. The constant pressure piston chamber 6 communicates through a passage 8a in the piston l with an annular recess 9 in the cylindrical peripheral surface of the piston. The recess 9 is located to remain in communication with an annular port 10 in the piston cylinder 2 during reciprocation of the piston. The port 10 communicates through a passage 11 with a pressure port 12 which is intended for connection to a source of hydraulic fluid under pressure. The alternating pressure piston chamber 8 communicates through a passage 13 to an alternating port 14 which is connectable by way of control means W0 alternately to a source of hydraulic fluid under pressure and exhaust.

it will be apparent that with the pressure port 12 connected to a source of hydraulic fluid under pressure and the alternating port 14 connected to exhaust the piston 1 will move rightwardly in the drawing (that is to say in the direction of its bias() due to the effect of hydraulic pressure in the chamber 6 and exhaust in the chamber 8. The piston will eventually reach the end of its stroke in the direction of its bias and will be held in this position by pressure in the chamber 6. If the alternating port 14 is now connected to the source of hydraulic fluid under pressure, pressure in the chamber 8 causes the piston l to move in a direction against its bias (that is leftwardly in the drawing) until it abuts the housing 3 at the end of its stroke and remains there by the action of the fluid pressure in the chamber 8. By alternately connecting the chamber 8 to exhaust and hydraulic fluid under pressure the piston l is caused to reciprocate as required. Reciprocation of the piston l is conveniently controlled by use of a spool or like valve which is adapted to reciprocate in phase with the reciprocation of the piston so that reciprocation of the spool valve controls reciprocation of the piston and auxiliary ports and control surfaces provided between the piston i and its cylinder 2 can be adapted to control reciprocation of the spool valve. Preferably the cyclic operation between the piston and spool valve is arranged to operate in the manner disclosed in one of our aforementioned copending patent applications. it will be realized however, that controlled reciprocation (either intermittent or continuous) of the piston can be achieved by any means which is well known in the relevant art.

The housing 3 further defines a spool cylinder 15 within which is slidably mounted a cylindrical spool (or auxiliary piston) :16. The spool 16 is provided with a spool rod 17 which extends through the housing 3. The longitudinal axis of the spool cylinder 1.5 is substantially radially disposed relative to the piston cylinder 2 and the spool cylinder 315 communicate; at one end with the piston cylinder 2. The spool 16 is capable of sliding movement through its cylinder 15 so that one end of the spool can enter the piston cylinder 2 and engage with a cam surface 18 provided on the piston l. The end of the spool 16 which engages with the cam surface 18 is in the form of a cylindrical roller 19 which is pinned to the end of the spool 16 for rotation thereon. The cam surface 18 is in the form of a flat (see FIG. 2) which is machined to be inclined relative to the longitudinal axis of the piston I so that the engagement between the flat and roller 19 inhibits the piston 1 from axially rotating in its cylinder 2.

The end of the spool 16 adjacent the spool rod 17 defines with its cylinder 315 a constant pressure spool chamber 20. The

constant pressure spool chamber 20 communicates through a passage 21 in the housing 3 with the constant pressure piston chamber 6. The end of the spool which communicates with the constant pressure spool chamber 20 is, in effect, the area of the annulus formed by the shoulder 22 between the cylindrical surface of the spool 16 and the cylindrical surface of the spool rod 17. The chamber 20 is adapted to be in constant communication with hydraulic fluid under pressure in a similar manner to the chamber 6 and this is effected by way of the passage 211, chamber 6, passage 3a, recess 9, port 110, passage Ill and pressure port l2. With the constant pressure spool chamber 2% under pressure, the spool 16 is urged into the spool cylinder 2 and the roller 19 into engagement with the cam surface l8 by the biasing effect of the hydraulic pressure on the working pressure area 22 of the spool.

it will be apparent that as the piston ll reciprocates in an axial direction, the spool 16 is caused to reciprocate in its cylinder and the locus of movement of the free end of the spool rod 117 is dependent upon the longitudinal profile of the cam surface 18 and the stroke of the piston. Consequently a required tool can be attached to the end of the piston rod 4 and the free end of the spool rod 17 secured to a frame shown generally at Mill. in operation of the device the piston reciprocates in a first axial sense and the housing 3 reciprocates on the relatively fixed spool T6 in a second axial sense which is normal to the first axial sense and a compound motion is thereby imparted to the end i of the piston rod. Clearly the path of such compound motion can be varied as required by alteration of the cam surface 18.

The part of the piston cylinder 2 with which the end of the spool 16 communicates is open to exhaust by way of a passage 26 so that fluid at the end of the spool to in cylinder 2 which results by leakage from chambers 6 and 20 can escape to exhaust and thereby the possibility of back pressure on the spool is prevented.

The effect of hydraulic pressure in the constant pressure spool chamber 20 not only urges the spool into engagement with the piston l but also tends to radially displace the piston l in its cylinder 2 and thereby increase the friction between the piston and cylinder on the side of the piston remote from the cylindrical roller 19. Such displacement can result in seizure of the piston. To overcome the possibility of such seizure the motor includes thrust means to maintain the piston in balance in a radial sense. Thepiston l is provided with a part annular recess 23 which is located substantially diametrically opposite to the cam surface l8 within which recess 23 is intended to be formed a hydrostatic pad located between the piston and its cylinder. The hydrostatic pad constitutes the thrust means and biases the piston in a direction opposite to the force applied thereto by the spool M to maintain the piston in balance. In the embodiment shown in H68. 1 and 2 the recess 23 communicates with the passage ha so that the hydraulic fluid pressure in the recess 23 is approximately equal to the pressure in chambers b and 20 and in this case the area of the recess 23 which opens to the piston cylinder 2 is equal to the working pressure area 22 of the spool lid so that the biasing force on the spool which tends to radially displace the piston is substantially opposed by an equal force applied by the thrust means which maintains the piston in balance.

in the modification shown in H6. 3, to overcome the possibility of seizure between the piston and its cylinder, the thrust means in the form of a hydrostatic pad is adapted to be formed between the piston and its cylinder in a similar manner to the construction shown in FIG. l but the pad is located in a part annular recess 24 formed in the wall of the larger diameter part of the piston cylinder 2. The recess M is located substantially diametrically opposite to the roller 9 to be in permanent communication with the peripheral surface of the piston l and is in permanent communication by way of passage 25 with the constant pressure piston chamber 6 so that hydraulic pressure in recess 2d is approximately equal to the pressure in chambers b and 20. The area of recess 24 which opens to the peripheral surface of the piston l is equal to the working pressure area 22 so that substantially the same antiseizure conditions prevail for the construction shown in FIG. 3 to those for the construction shown in FlGS. l and 2.

What we claim is:

l. A fluid pressure operated reciprocatory motor, comprising: a piston axially slidable in a piston cylinder, said piston having opposed working faces of different effective areas, its smaller effective area communicating with a constant pressure piston chamber in permanent communication with a source of fluid pressure to bias said piston in one sense of axial direction, and its larger effective area communicating with an alternating pressure piston chamber; control means alternate ly connecting said alternating pressure piston chamber with a source of fluid pressure and with exhaust for imparting reciprocatory movement to said piston in its cylinder; a spool axially slidable in a spool cylinder, said spool cylinder being fixedly located with respect to said piston cylinder with the axis of said spool cylinder disposed substantially radially with respect to said piston cylinder and said spool cylinder communicating with said piston cylinder; means biasing said spool axially in its cylinder towards said piston cylinder; a cam surface provided on said piston; an abutment surface provided on said spool, said abutment surface abutting said cam surface and being urged into engagement therewith by said biasing means whereby, during reciprocation of said piston, said abutment surface rides over said cam surface to impart axial movement to said spool; and wherein thrust means is located between said piston and said piston cylinder for providing a radially directed force on said piston which force opposes, and is substantially equal to, the radial force exerted on said piston by said biasing means through said spool.

2. A motor as claimed in claim ll, further comprising recess means provided between said piston and piston cylinder, said thrust means being adapted to comprise fluid pressure pad means located by and forced in said recess means; and passage means which communicates between said constant pressure piston chamber and said recess means for supplying fluid under pressure between said recess means and said constant pressure piston chamber.

3. A motor as claimed in claim 1, in which said biasing means is adapted to be provided by fluid under pressure acting on said spool and wherein said spool has a working face which communicates with a spool chamber and passage means is provided which communicates between said constant pressure piston chamber and said spool chamber for supplying fluid under pressure between said spool chamber and said constant pressure piston chamber whereby said spool is biased axially under such fluid pressure to urge said abutment and cam surfaces into engagement.

4. A fluid pressure operated reciprocatory motor, comprising: a substantially cylindrical piston axially slidable in a piston cylinder; means restraining rotation of said piston about its axis relative to said piston cylinder; fluid pressure operated means imparting reciprocatory movement to said piston in its cylinder; a spool axially slidable in a spool cylinder, said spool cylinder being fixedly located with respect to said piston cylinder with the axis of said spool cylinder disposed substantially radially with respect to said piston cylinder and said spool cylinder communicating with said piston cylinder; means biasing said spool axially in its cylinder towards said piston cylinder; a cam surface provided on said piston, said cam surface having a flat extending axially of said piston in a predetermined profile with respect to the piston axis, the abutment surface of said spool engaging with said flat to restrain rotation of said piston in said piston cylinder; an abutment surface provided on said spool, said abutment surface abutting said cam surface and being urged into engagement therewith by said biasing means whereby, during reciprocation of said piston, said abutment surface rides over said cam surface to impart axial movement to said spool; and wherein thrust means is located between said piston and said piston cylinder for providing a radially directed force on said piston which force opposes, and is substantially equal to, the radial force exerted on the said piston by said biasing means through said spool.

5. A fluid pressure operated reciprocatory motor, comprising a piston axially slidable in a piston cylinder; fluid pressure operated means imparting reciprocatory movement to said piston in its cylinder; a spool axially slidable in a spool cylinder, said spool cylinder being fixedly located with respect to said piston cylinder with the axis of said spool cylinder disposed substantially radially with respect to said piston cylinder and said spool cylinder communicating with said piston cylinder; means biasing said spool axially in its cylinder towards said piston cylinder; a cam surface provided on said piston; an abutment surface provided on said spool, said abutment surface abutting said cam surface and being urged into engagement therewith by said biasing means whereby, during reciprocation of said piston, said abutment surface rides over said cam surface to impart axial movement to said spool; and wherein thrust means is located between said piston and said piston cylinder for providing a radially directed force on said piston which force opposes, and is substantially equal to, the radial force exerted on said piston by said biasing means through said spool; and further comprising in combination with a frame, a housing in which said spool cylinder and piston cylinder are commonly defined, and wherein said spool extends through its cylinder from said housing and is attached to said frame to mount said motor on said frame whereby, during reciprocation of said piston in its cylinder, said housing ex hibits movement relative to said spool and frame in the direction of the axis of said spool and an end of said piston exhibits a compound motion relative to said frame as a result of reciprocation of said piston in the direction of its axis and movement of said housing in the direction of the axis of said spool.

6. A fluid pressure operated reciprocatory motor, comprising: a piston axially slidable in a piston cylinder; fluid pressure operated means imparting reciprocatory movement to said piston in its cylinder; a spool axially slidable in a spool cylinder, said spool cylinder being fixedly located with respect to said piston cylinder and with the axis of said spool cylinder disposed substantially radially with respect to said piston cylinder; a cam surface provided on said piston; an abutment surface provided on said spool, said abutment surface abutting said cam surface; means biasing said spool axially in its cylinder and said abutment surface into engagement with said cam surface, whereby, during reciprocation of said piston, said abutment surface rides over said cam surface to impart axial movement to said spool; a working face on said spool communicating with a spool chamber; means connecting said spool chamber to a source of fluid pressure, said fluid pressure providing said biasing means to act on said working face; recess means between said piston and said piston cylinder and located on the side of said piston remote from the position of engagement between said cam surface and said abutment surface, said recess means providing a location for fluid pressure pad means and having an effective area which is substantially equal to the area of said spool working face; and passage means communicating between said recess means and said spool chamber through which fluid pressure in said spool chamber and said recess means is equa ized whereby, with fluid pressure in said spool chamber and in said recess means, radially directed forces on said piston applied by said abutment surface of the spool and fluid pressure pad means formed in said recess means are substantially equal and p posite.

7. A motor as claimed in claim 6, in which said recess means comprises a part annular recess defined in said piston cylinder and located in diametrical alignment with said position of engagement between said cam surface and said abutment surface,

3. A motor as claimed in claim 6, in which said recess means comprises a part annular recess defined in the peripheral surface of said piston and located in diametrical alignment with said cam surface.

9. A motor as claimed in claim 6, wherein said piston has opposed working faces of different effective areas, its smaller effective area communicating with a constant pressure piston chamber in permanent communication with a source of fluid pressure to bias said piston in one sense of axial direction and its larger effective area communicating with an alternating pressure piston chamber; said motor further comprising control means alternately connecting said alternating pressure piston chamber with a source of fluid pressure and with exhaust to reciprocate said piston.

10. A motor as claimed in claim 9, wherein said recess means and said spool chamber communicate by way of passage means with said constant pressure piston chamber.

H. A motor as claimed in claim 6, in which said piston is substantially cylindrical and has a cam surface comprising a flat extending axially of said piston in predetermined profile with respect to the piston axis, the abutment surface of said spool engaging with said flat to restrain rotation of said piston in said piston cylinder.

12. A motor as claimed in claim 6, comprising in combination with a frame, a housing in which said spool cylinder and piston cylinder are commonly defined, and wherein said spool extends through its cylinder from said housing and is attached to said frame to mount said motor on said frame whereby, during reciprocation of said piston in its cylinder, said housing moves relative to said spool and frame in the direction of the axis of said spool and an end of said piston exhibits a compound motion relative to said frame as a result of reciprocation of said piston in the direction of its axis and movement of said housing in the direction of the axis of said spool.

13. A fluid pressure operated reciprocatory motor which comprises: a piston axially slidable in a piston cylinder; said piston including fluid pressure operated means imparting reciprocatory movement to said piston in its cylinder; a spool axially slidable in a spool cylinder, said spool cylinder being fixedly located with respect to said piston cylinder and with the axis of said spool cylinder disposed substantially radially with respect to said piston cylinder; a cam surface provided on said piston; an abutment surface provided on said spool, said abutment surface abutting said cam surface; means biasing said spool axially in its cylinder and said abutment surface into engagement with said cam surface whereby, during reciprocation of said piston, said abutment surface rides over said cam surface to impart axial movement to said spool; thrust between said piston and said piston cylinder located on the side of said piston remote from the position of engagement between said cam surface and said abutment surface, said thrust means providing a radially directed force on said piston which opposes the radially directed force on said piston applied by said abutment surface of the spool; and means interconnecting said biasing means and said thrust means c. maintaining the radially directed force applied on said piston by said thrust means substantially equal to the opposed radially directed force applied on the piston by said biasing means through said abutment surface. 

1. A fluid pressure operated reciprocatory motor, comprising: a piston axially slidable in a piston cylinder, said piston having opposed working faces of different effective areas, its smaller effective area communicating with a constant pressure piston chamber in permanent communication with a source of fluid pressure to bias said piston in one sense of axial direction, and its larger effective area communicating with an alternating pressure piston chamber; control means alternately connecting said alternating pressure piston chamber with a source of fluid pressure and with exhaust for imparting reciprocatory movement to said piston in its cylinder; a spool axially slidable in a spool cylinder, said spool cylinder being fixedly located with respect to said piston cylinder with the axis of said spool cylinder disposed substantially radially with respect to said piston cylinder and said spool cylinder communicating with said piston cylinder; means biasing said spool axially in its cylinder towards said piston cylinder; a cam surface provided on said piston; an abutment surface provided on said spool, said abutment surface abutting said cam surface and being urged into engAgement therewith by said biasing means whereby, during reciprocation of said piston, said abutment surface rides over said cam surface to impart axial movement to said spool; and wherein thrust means is located between said piston and said piston cylinder for providing a radially directed force on said piston which force opposes, and is substantially equal to, the radial force exerted on said piston by said biasing means through said spool.
 2. A motor as claimed in claim 1, further comprising recess means provided between said piston and piston cylinder, said thrust means being adapted to comprise fluid pressure pad means located by and forced in said recess means; and passage means which communicates between said constant pressure piston chamber and said recess means for supplying fluid under pressure between said recess means and said constant pressure piston chamber.
 3. A motor as claimed in claim 1, in which said biasing means is adapted to be provided by fluid under pressure acting on said spool and wherein said spool has a working face which communicates with a spool chamber and passage means is provided which communicates between said constant pressure piston chamber and said spool chamber for supplying fluid under pressure between said spool chamber and said constant pressure piston chamber whereby said spool is biased axially under such fluid pressure to urge said abutment and cam surfaces into engagement.
 4. A fluid pressure operated reciprocatory motor, comprising: a substantially cylindrical piston axially slidable in a piston cylinder; means restraining rotation of said piston about its axis relative to said piston cylinder; fluid pressure operated means imparting reciprocatory movement to said piston in its cylinder; a spool axially slidable in a spool cylinder, said spool cylinder being fixedly located with respect to said piston cylinder with the axis of said spool cylinder disposed substantially radially with respect to said piston cylinder and said spool cylinder communicating with said piston cylinder; means biasing said spool axially in its cylinder towards said piston cylinder; a cam surface provided on said piston, said cam surface having a flat extending axially of said piston in a predetermined profile with respect to the piston axis, the abutment surface of said spool engaging with said flat to restrain rotation of said piston in said piston cylinder; an abutment surface provided on said spool, said abutment surface abutting said cam surface and being urged into engagement therewith by said biasing means whereby, during reciprocation of said piston, said abutment surface rides over said cam surface to impart axial movement to said spool; and wherein thrust means is located between said piston and said piston cylinder for providing a radially directed force on said piston which force opposes, and is substantially equal to, the radial force exerted on the said piston by said biasing means through said spool.
 5. A fluid pressure operated reciprocatory motor, comprising a piston axially slidable in a piston cylinder; fluid pressure operated means imparting reciprocatory movement to said piston in its cylinder; a spool axially slidable in a spool cylinder, said spool cylinder being fixedly located with respect to said piston cylinder with the axis of said spool cylinder disposed substantially radially with respect to said piston cylinder and said spool cylinder communicating with said piston cylinder; means biasing said spool axially in its cylinder towards said piston cylinder; a cam surface provided on said piston; an abutment surface provided on said spool, said abutment surface abutting said cam surface and being urged into engagement therewith by said biasing means whereby, during reciprocation of said piston, said abutment surface rides over said cam surface to impart axial movement to said spool; and wherein thrust means is located between said piston and said piston cylinder for providing a radially directed force on said pistoN which force opposes, and is substantially equal to, the radial force exerted on said piston by said biasing means through said spool; and further comprising in combination with a frame, a housing in which said spool cylinder and piston cylinder are commonly defined, and wherein said spool extends through its cylinder from said housing and is attached to said frame to mount said motor on said frame whereby, during reciprocation of said piston in its cylinder, said housing exhibits movement relative to said spool and frame in the direction of the axis of said spool and an end of said piston exhibits a compound motion relative to said frame as a result of reciprocation of said piston in the direction of its axis and movement of said housing in the direction of the axis of said spool.
 6. A fluid pressure operated reciprocatory motor, comprising: a piston axially slidable in a piston cylinder; fluid pressure operated means imparting reciprocatory movement to said piston in its cylinder; a spool axially slidable in a spool cylinder, said spool cylinder being fixedly located with respect to said piston cylinder and with the axis of said spool cylinder disposed substantially radially with respect to said piston cylinder; a cam surface provided on said piston; an abutment surface provided on said spool, said abutment surface abutting said cam surface; means biasing said spool axially in its cylinder and said abutment surface into engagement with said cam surface, whereby, during reciprocation of said piston, said abutment surface rides over said cam surface to impart axial movement to said spool; a working face on said spool communicating with a spool chamber; means connecting said spool chamber to a source of fluid pressure, said fluid pressure providing said biasing means to act on said working face; recess means between said piston and said piston cylinder and located on the side of said piston remote from the position of engagement between said cam surface and said abutment surface, said recess means providing a location for fluid pressure pad means and having an effective area which is substantially equal to the area of said spool working face; and passage means communicating between said recess means and said spool chamber through which fluid pressure in said spool chamber and said recess means is equalized whereby, with fluid pressure in said spool chamber and in said recess means, radially directed forces on said piston applied by said abutment surface of the spool and fluid pressure pad means formed in said recess means are substantially equal and opposite.
 7. A motor as claimed in claim 6, in which said recess means comprises a part annular recess defined in said piston cylinder and located in diametrical alignment with said position of engagement between said cam surface and said abutment surface.
 8. A motor as claimed in claim 6, in which said recess means comprises a part annular recess defined in the peripheral surface of said piston and located in diametrical alignment with said cam surface.
 9. A motor as claimed in claim 6, wherein said piston has opposed working faces of different effective areas, its smaller effective area communicating with a constant pressure piston chamber in permanent communication with a source of fluid pressure to bias said piston in one sense of axial direction and its larger effective area communicating with an alternating pressure piston chamber; said motor further comprising control means alternately connecting said alternating pressure piston chamber with a source of fluid pressure and with exhaust to reciprocate said piston.
 10. A motor as claimed in claim 9, wherein said recess means and said spool chamber communicate by way of passage means with said constant pressure piston chamber.
 11. A motor as claimed in claim 6, in which said piston is substantially cylindrical and has a cam surface comprising a flat extending axially of said piston in predetermined profile with respect to the piston axis, the abutment surface of said spool engaging with said flat to restrain rotation of said piston in said piston cylinder.
 12. A motor as claimed in claim 6, comprising in combination with a frame, a housing in which said spool cylinder and piston cylinder are commonly defined, and wherein said spool extends through its cylinder from said housing and is attached to said frame to mount said motor on said frame whereby, during reciprocation of said piston in its cylinder, said housing moves relative to said spool and frame in the direction of the axis of said spool and an end of said piston exhibits a compound motion relative to said frame as a result of reciprocation of said piston in the direction of its axis and movement of said housing in the direction of the axis of said spool.
 13. A fluid pressure operated reciprocatory motor which comprises: a piston axially slidable in a piston cylinder; said piston including fluid pressure operated means imparting reciprocatory movement to said piston in its cylinder; a spool axially slidable in a spool cylinder, said spool cylinder being fixedly located with respect to said piston cylinder and with the axis of said spool cylinder disposed substantially radially with respect to said piston cylinder; a cam surface provided on said piston; an abutment surface provided on said spool, said abutment surface abutting said cam surface; means biasing said spool axially in its cylinder and said abutment surface into engagement with said cam surface whereby, during reciprocation of said piston, said abutment surface rides over said cam surface to impart axial movement to said spool; thrust means between said piston and said piston cylinder and located on the side of said piston remote from the position of engagement between said cam surface and said abutment surface, said thrust means providing a radially directed force on said piston which opposes the radially directed force on said piston applied by said abutment surface of the spool; and means interconnecting said biasing means and said thrust means and maintaining the radially directed force applied on said piston by said thrust means substantially equal to the opposed radially directed force applied on the piston by said biasing means through said abutment surface. 